1. Field of the Invention
The present invention relates to a gooseneck condenser microphone in which a microphone body is supported by a flexible supporting pipe, and more particularly, to a technique capable of preventing the generation of noise due to electromagnetic waves emitted from, for example, a mobile phone.
2. Description of the Related Art
A gooseneck condenser microphone (hereinafter, simply referred to as a “condenser microphone”) has a simple appearance and makes it easy to adjust an angle or a height with respect to the speaker. Therefore, the gooseneck condenser microphone is widely used in, for example, conference facilities, such as international conference halls, or TV studios.
The gooseneck condenser microphones are mainly classified into a separation type in which the microphone unit and the output module are separated from each other and an integrated type in which the microphone unit and the output module are connected to each other.
That is, in a separation-type condenser microphone A illustrated in FIG. 4, a condenser microphone unit (hereinafter, simply referred to as a “microphone unit”) 10 is separated from an output module 20 including an audio signal output circuit board 21 for the microphone unit, the microphone unit 10 is supported at the leading end of a supporting pipe 30, and the output module 20 is attached to the rear end (base) of the supporting pipe 30.
The supporting pipe 30 includes a flexible shaft 31. In this example, the flexible shaft 31 includes a front flexible shaft 31a, a rear flexible shaft 31b, and a relay pipe 32 which is a metal straight pipe and is interposed between the front and rear flexible shafts. The output module 20 includes a shielded housing 20a and is provided on a base, such as a table, through a fixing bracket (not illustrated).
The microphone unit 10 and the audio signal output circuit board 21 of the output module 20 are electrically connected to each other by a microphone cable 40 inserted into the supporting pipe 30. A two-core shielded line is used as the microphone cable 40. In the separation type, the audio signal output circuit board 21 and an output connector 22 are provided in the output module 20.
In general, as the output connector 22, an output connector is used which includes a first pin for ground, a second pin which is on the hot side of a signal, and a third pin which is on the cold side, which are specified in EIAJ RC-5236 “Latch Lock Type Round Connector for Audio Equipment”.
Although not illustrated in the drawings, the microphone unit 10 includes an FET (field effect transistor) as an impedance converter. In the separation type, the microphone cable 40 is an unbalanced transmission table. In the microphone unit 10, one core of the microphone cable 40 is connected as a power line to the drain of the FET and the other core thereof is connected as a signal line to the source. In addition, the shielded line is connected to a unit case which is the ground. The source of the FET is also connected to the unit case (ground).
In the output module 20, the power supply side of the microphone cable 40 and the signal line are connected to predetermined terminals of the audio signal output circuit board 21 and the shielded line is connected to the ground (ground circuit) of the audio signal output circuit board 21. The ground of the audio signal output circuit board 21 is connected to the first pin of the output connector 22, and the first pin is also connected to the shielded housing 20a of the output module 20. That is, the first pin is the base point of ground.
The output connector 22 is connected to a phantom power supply (not illustrated) through a balanced two-core shielded cable. In some cases, the output module 20 is referred to as a power module since it supplies power to the microphone unit 10.
In contrast, an integrated condenser microphone B illustrated in FIG. 5 includes a microphone body M which connects the microphone unit 10 and the output module 20. The microphone body M is supported at the leading end of the supporting pipe 30. A base housing 50 including only the output connector 22 is attached to the rear end of the supporting pipe 30.
In the condenser microphone B, the audio signal output circuit board 21 in the output module 20 and the output connector 22 in the base housing 50 are electrically connected to each other through the microphone cable 40.
In the integrated type, the microphone cable 40 is a balanced transmission cable. In the output module 20, a hot-side signal line and a cold-side signal line of the microphone cable 40 are connected to the drain and source of the FET through predetermined wiring lines of the audio signal output circuit board 21, and a shielded line is connected to the ground of the audio signal output circuit board 21. However, the source of the FET and the ground of the audio signal output circuit board 21 are connected to the shielded housing 20a which serves as the ground.
In the base housing 50, the hot-side signal line and the cold-side signal line of the microphone cable 40 are connected to the second and third pins of the output connector 22 and the shielded line is connected to the first pin. The first pin is also connected to the base housing 50. In the condenser microphone B, the first pin is the base point of the ground.
In this example, the supporting pipe 30 includes a flexible shaft 31 which is provided at the rear end and a relay pipe 32. A coupler (connection member) 20b made of a metal material is provided at the rear end of the shielded housing 20a of the output module 20. The output module 20 is connected to the relay pipe 32 through the coupler 20b. 
However, in both the separation-type condenser microphone A and the integrated condenser microphone B, the supporting pipe 30 and the shielded line of the microphone cable 40 function as an antenna and are likely to be affected by external noise (disturbance electromagnetic waves).
The flexible shaft 31 includes a steel coil spring and a triangular wire rod which is made of, for example, a copper alloy and is plastically deformed. A contact portion between the wire rods has impedance although the resistance value thereof is small (for example, about 1Ω). Therefore, the microphone cable 40 is not completely shielded from a high frequency.
When a strong disturbance electromagnetic wave is emitted to the microphone cable 40, it is transmitted as a high-frequency current to the microphone unit 10 or the output module 20 and is detected by a semiconductor device, such as an FET. As a result, noise is generated due to the high-frequency current.
In particular, a considerably strong electromagnetic wave (for example, electric field intensity which is tens to thousands of times more than that generated by commercial radio waves in the range of several centimeters to several tens of centimeters) is emitted from the mobile phone. Therefore, in the field of a condenser microphone, there is an urgent need to take measures for electromagnetic waves when the mobile phone is used at a short distance.
For the separation-type condenser microphone A illustrated in FIG. 4, in Japanese Patent Application Laid-Open (JP-A) No. 2006-33216, the present applicant discloses a structure in which the external sheath of at least a portion which is disposed inside a flexible shaft in the microphone cable inserted into a supporting pipe is removed such that a shielded line is exposed and the shielded line is electrically connected to the flexible shaft at multiple points.
According to this structure, it is expected that the resistance value of the flexible shaft will be significantly reduced, a shielding function for electromagnetic waves will be improved, and the generation of noise due to disturbance electromagnetic waves will be effectively prevented.
The invention disclosed in JP-A No. 2006-33216 is a little effective for the integrated condenser microphone B illustrated in FIG. 5. However, in the integrated type, since the distance between the ground (ground circuit) of the audio signal output circuit board 21 in the output module 20 and the ground base point (first pin) of the entire microphone is equal to the length of the supporting pipe 30, a high-frequency current generated due to the disturbance electromagnetic wave is more likely to be mixed than that in the separation type.
In addition, since the supporting pipe 30 is electrically connected to the shielded housing 20a of the output module 20, the disturbance electromagnetic wave captured by the supporting pipe 30 is transmitted as a high-frequency current from the shielded housing 20a to the output module 20.
In order to solve the above-mentioned problems, the present applicant has provided a gooseneck condenser microphone in which a microphone case (shielded housing) is insulated from a supporting pipe to prevent the mixture of a high-frequency current with an output module (Japanese Patent Application No. 2010-035903).
The structure of a condenser microphone C disclosed in Japanese Patent Application No. 2010-035903 will be described with reference to FIG. 6. In FIG. 6, the same components as those in the integrated condenser microphone illustrated in FIG. 5 are denoted by the same reference numerals.
The condenser microphone C illustrated in FIG. 6 includes a metal cover 60 which is a gold-plated brass member and is provided at the rear end of a shielded housing 20a in order to shield the disturbance electromagnetic wave. A core insertion hole 63 is formed in the metal cover 60 and cores 41 and 42 of a microphone cable 40 are inserted into a shielded housing 20a through the core insertion hole 63 and are connected to predetermined terminals of an audio signal output circuit board 21 by, for example, soldering. In addition, a shielded line 43 of the microphone cable 40 is connected to the metal cover 60 by, for example, soldering and the metal cover 60 is electrically connected to a ground pattern of the audio signal output circuit board 21. A cylindrical electrical insulating member 80 made of a resin (for example, an ABS resin) is provided between a coupler 20b and a relay pipe 32 (supporting pipe 30) such that the coupler 20b and the relay pipe 32 are electrically insulated from each other. The coupler 20b is inserted into the shielded housing 20a and is electrically connected to the shielded housing 20a. 
According to this structure, a complete shield in which a contact portion between the metal cover 60 and the shielded housing 20b is the base point of ground is formed in the output module 20. Therefore, it is possible to prevent the mixture of the disturbance electromagnetic wave captured by the supporting pipe 30.
However, in the structure illustrated in FIG. 6, the relay pipe 21 (supporting pipe 30) is simply inserted into the coupler 20b with the electrical insulating member 80 interposed between and the interposed electrical insulating member 80 is made of a resin. Therefore, the electrical insulating member 80 made of a resin is gradually plastically deformed in the thickness direction and the thickness thereof is reduced. As a result, the coupling performance of the connection portion is reduced.